The invention is concerned with improvements in the thermal cracking of hydrocarbons, such as ethane, propane, butane, naphtha, or gas oil to form olefins, such as ethylene, propylene, or butenes. It is particularly concerned with avoiding, or at least lessening, the formation of carbon deposits, commonly referred to as coke, on a reactor element wall during a thermal cracking process.
At the heart of a thermal cracking process is the pyrolysis furnace. This furnace comprises a fire box through which runs a serpentine array of tubing. This array is composed of lengths of tubing and fittings that may total several hundred meters in length. The array of tubing is heated to a carefully monitored temperature by the fire box. A stream of feedstock is forced through the heated tubing under pressure and at a high velocity, and the product quenched as it exits. For olefin production, the feedstock is frequently diluted with steam. The mixture is passed through the tubing array which is commonly operated at a temperature greater than 750/C. During this passage, a carboniferous residue is formed and deposits on the tube walls and fittings.
Initially, carbon residue appears in a fibrous form on the walls. It is thought this results from a catalytic action, primarily due to nickel and iron in the tube metal. The carbon fibers on the tube wall appear to form a mat which traps pyrolitic coke particles formed in the gas stream. This leads to build-up of a dense, coke deposit on the walls of the tubing and fittings.
The problem of carbon deposits forming during the thermal cracking of hydrocarbons is one of long standing. It results in restricted flow of the gaseous stream of reaction material. It also reduces heat transfer through the tube wall to the gaseous stream. The temperature to which the tube is heated must then be raised to maintain a constant temperature in the stream flowing through the tube. This not only reduces process efficiency, but ultimately requires a temperature too high for equipment viability, as well as safety requirements. A shutdown then becomes necessary to remove the carbon formation, a process known as decoking.
Numerous solutions to the problem of coking have been proposed. One such solution involves producing metal alloys having special compositions. Another proposed solution involves coating the interior wall of the tubing with a silicon-containing coating, such as silica, silicon carbide, or silicon nitride. In still another proposal, the interior wall of the tubing is treated with an aluminum compound. This process involves aluminum surface conversion as well as diffusion into the metal. It has also been proposed to introduce additives, such as sulfides, to the feedstock stream.
Despite these numerous proposals, the problem still remains. It is then a basic purpose of the present invention to provide an effective solution to the problem.